Abstract: The magnetically controlled narrow-gap TIG welding experiment was conducted for the 120 mm thick TA17 titanium alloy plate.The microstructure and mechanical properties of different regions of the welded joint were characterized systematically, and the evolution process of the microstructure and its influence on the mechanical properties were clarified.The results show that the microstructures of the joint in the transverse direction have significant difference.The base metal area near the weld zone is mainly composed of elongated α grain and β phase which is distributed along grain boundary.The heat affected zone is the Widmannian structure composed of large α cluster, and the fusion zone is a net basket structure composed of interlaced α lamellar.In the thickness direction of the joint, there is almost no difference in the microstructure, and a small amount of martensite lamellar microstructure is observed in the upper fusion zone of the joint.The hardness of the fusion zone is significantly lower than that of the base metal zone.During tensile process, the fusion zone yields first, and then becomes the area where the plastic deformation concentrates, resulting in the decrease of the joint strength and plasticity.